This invention relates to the preparation of refractory metal boride powders, particularly to the preparation of such powders with a predetermined particle size.
Refractory metal boride compounds are important industrial materials. They possess high melting points, high hardnesses, low electrical resistivities, high thermal conductivities, and moderately low specific gravities. In addition, several of these compounds are resistant to oxidation and are chemically inert in many harsh, corrosive environments. Because of the above properties, these compounds are used in cutting and grinding tools, electrodes in harsh environments, and armor plating.
Boride compounds are often produced as fine powders, and dense products can be made from these powders using known forming operations, for example, hot pressing or sintering processes. Powders which are best suited for such processes are very small particles with high surface energies which are capable of being consolidated at high temperatures.
There are a number of methods of producing boride powders, and these manufacturing processes include direct combination of a metal or its hydride with boron at high temperatures, reduction of a metal oxide with boron or a mixture of boron and carbon, reaction of a boron halide with a metal or metal oxide and hydrogen, and fused salt electrolysis of the metal oxide with a source of boron oxide. However, the most widely used method is the carbothermal reduction described, for example, in U.S. Pat. Nos. 2,957,754 and 3,328,127. This method involves the reaction of titanium dioxide with boron oxide or boron carbide in the presence of carbon at high temperatures. While this method is widely used, it has two major drawbacks. First, the use of high temperature leads to the formation of titanium diboride with very large grain sizes. Second, control of the particle size is difficult because the product crystallizes rapidly and these crystals grow rapidly under the reaction conditions. These particles then require size reduction before use in most forming operations, which results in (1) impurities from the size reduction process, (2) irregularly shaped and fractured particles, and (3) a very large particle size distribution.
In many applications, particularly those involving sintering or hot pressing operations, the boride compounds should be of controlled and, preferably, submicron size. One method of producing such particles is through a gas phase reaction described, for example, in U.S. Pat. No. 4,503,021. In this method, gaseous boron halide is reacted with titanium halide and hydrogen to produce titanium diboride and hydrogen halide. Although some starting reactants may, in some cases, be solid, the boron halide must be in the gas phase.
In view of the above limitations in the present methods of producing metal boride compounds, it is clear that new processes are desirable to allow for more flexible manufacture. A method is especially desirable which would allow the manufacture of boride powders of a predetermined particle size, ranging from submicron-sized up to a diameter of about 50 microns.